Telephone substation circuit



e R. H. HERRICK 2,369,144

TELEPHONE SUBSTATION C IRCUIT File d May 10, 1945 s Sheets-Shegt 1 l2 FIG. I

- I8 l7 IIs HYBRID o 0R L; BALANCING g o NETWORK 90 I3 I C J AMPLIFIER INVENTOR. ROSWELL H. HERRICK I ml ywom ATTORNEYS on M23 3 Sheets-Sheet 2 F IG'. 2

INVENTOR.

ROSWELL H. HERRICK HYBRID OR BALANCING NETWOFH AMPLIFIER R. H. HERRICK Filed May 10, 1 43 TELEPHONE SUBSTATION CIRCUIT Feb. 13, 1945.

ATTORNEYS Feb. 13, 1945.

R. H. HERRICK 2,369,144

TELEPHONE SUBSTATION CIRCUIT Filed May 10, 1945 S'Sheet's-Shet 3 LPF 65 no S 7 3 m 2 9 HYBRID OR BALANCING. W n I v NETWORK 06 um- 33 0 I 35 f AMPLIFIER INVENTOR. ROSWELL H. HERRICK ATTORNEYS LINE 3O Patented Feb. 13, 1945 7 TELEPHONE SUBSTATION CIRCUIT Roswell H. Herrick, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Application May 10, 1942., Serial No. 486,383

17 Claims.

The present invention relates generally to improvements in telephone substation circuits having incorporated therein coupled signal current channels for the transmission of incoming and outgoing signal currents and, more particularly, to improvements in such substation circuits for preventing interaction between the signal channels during the calling interval in which the substation is calling another substation.

In numerous installations the telephone substation circuit is provided with incoming and outgoing channel amplifiers to provide loudspeaking telephone communication service. In the usual telephone substation circuit a hybrid system or anti-sidetone impedance network is provided for preventing signal, currentwdeveloped during the operation of the transmitter from being transmitted to the receiver and for similarly preventing signal currents incoming over the line extending to the substation from being transmitted to the transmitter for reproduction. In such an arrangement one of the factors which determines the efiiclency of sidetone suppression is the impedance of the'talking circuit established by way of twoconnected subscriber lines. Since this impedance is not the same for any two different established connections, the hybrid system of each substation is usually balanced to provide maximum sidetone suppression efficiency for average line conditions. In those substation circuits which are provided with amplifiers for incoming and outgoing signal currents, the conditions of unbalance introduced in the substation circuit by the impedances of the lines over which an established connection extends may become intolerable. This is particularly true in an exchange area where the subscriber lines are of widely different lengths. In order to obviate or minimize the interaction or singing which may result due to unbalance of the substation circuit occasioned by unfavorable line conditions, signal control switching means may be provided in the substation circuit for selectively blocking the channels when not in use. Such signal control switching means, however, are not eifective during that interval in which the substation subscriber is calling another substation. Accordingly it becomes apparent that the hybrid system or anti-sidetone impedance network is not effective until connection has been completed with a called subscriber station. This unbalance existing in the substation circuittherefore will permit an interaction between the incoming and outgoing channel amplifierswhich is commonly referred to as howling. While howling occurs only during the calling interval and will cease as soon as the called substation answers, this condition may be particularly annoying even though it does not persist for any great length of time and it does not impair communication between substations after the called substation has responded to the call. Accordingly it would be desirable to provide some means for obviating the possibility of interaction between the incoming and outgoing channel amplifiers during the calling period.

It is an object of the present invention to provide improved telephone substation apparatus of the character described, wherein at least one of the channel amplifiers is disabled sufliciently to prevent such interaction occurring between the channel amplifiers as to bring about a condition commonly known as howling.

It is another object of this invention to provide an improved telephone substation circuit with means for disabling at leastone of the channel amplifiers upon initiation of a call from the substation, together with means responsive to the completion of a connection at a called substation for terminating the aforementioned disabling action.

It is a further object of this invention to provide an improved telephone substation apparatus of the character describedwith means under the control of the dial tone, ring-back tone, or a dial calling device to disable at least one of the channel amplifiers during the calling in-' gether with means responsive to the upper range of voice frequency currents received from the called substation to restore the channel amplifier to normal operating condition.

A still further object of the present invention is to provide an improved telephone substation apparatus of the character described wherein one of the channel amplifiers is coupled to a control circuit for disabling at least one of the channel amplifiers during the calling interval, and in response to voice signals received from the called substation, the channel amplifiers are restored to normal operating condition and, the control circuit is disconnected from its associated channel amplifier for the entire communi 'it' period.

. The present invention is applicable to telephone substation circuits of'various types having incoming and outgoing channel amplifiers to provide loudspeaking telephone communication. Some substation circuits provide only loudspeaking telephone communication service, and others provide a selection between the loudspeaking telephone and the. conventional telephone. In some substation "circuits the amplifiers; are com pletely disabled or blocked when not in use, whereas in other arrangements the channel amplifiers when not in use are disabled to a certain degree. In each case the gain of the disabled channel amplifier is reduced sufficiently to prevent any appreciable interaction between the and to render ineffective the disabling apparatus.' In order to insure a relatively rapid restoration of the channel amplifiers to normal operating condition, still further circuit arrangements may be provided such as an'arrangement where- 4 -positively to its normal or unlocked position.

transmitter and the receiver at the-substation.

Various control circuits are also used to enable and disable the channel amplifiers in accordance with the direction of the flow of signal transmission. 1

cordance with the. present invention means are provided responsive to the initiation of a call for rendering all or part of the channel amplifiers inactive during this interval and means are provided responsive to the completion of a connection with a called substation for again restoring the operation oflthe channel amplifiers. At least one of the channel amplifiers may be disabled by applying thereto a bias 'sufiicient to reduce the gain level thereof or by a simple arrangement such as short-circuiting the transmitter,

the loudspeaker, or the input circuit of the out going channel amplifier. While certain advantages may be obtained by the application of one orthe other of the alternatives referred to, it is believed that these are' not necessary for a complete understanding of the present invention and, hence, these various methods have merely been indicated by showing an arrangement for shortcircuiting the transmitter. One method of .obtaining disablingresponsive to the initiation of a call is by the use of off-normal contacts on the dial calling device which, upon initialactuation of the dial, operate to lock in 'a relay which disables at least .one of the channel amplifiers until the called substation responds. High-pass filter means are coupled to theincoming channel. amplifier so that in response to an answer be rendered ineffective and normal operation of the channel amplifiers may thereby be restored.

80 The present invention is applicable. to any'ot from a called substation the disabling means may tone is interrupted during dialing, a dial device least one of the channel amplifiers until other means operating in response .to the higher. voice frequencies .receivedirom the calledsubstation become efiecti've to disconnect the control circuit Further features of the invention pertain to various arrangements of the circuit elements whereby the above and additional operating featuresare obtained.

The invention, both as to its organization and method of operation, together with further objectsandadvantages thereof, will be understood readilybyreierence to the following description taken in connection with the accompanying drawings in which Figure 1 illustrates a substation circuithaving incorporated therein certain of the features of the invention as briefly outlined above; 'Fig. 2 illustrates a substation circuit'arranged in a different manner than Fig. .1' and wherein the initial disabling action is brought about in response to the dial tone; and Fig. 3 shows a substation circuit similar to that shown in Fig. 2 which is provided with the additional feature of an arrangement for insuring a rapid restoration of the initial disabling relay to'lts I2, each provided with suitable channel ampli fiers I3 and I4 coupled to .ahybrid network or other'balancing circuit arrangement I5. Each of the channel amplifiers I3 and I4 may be provided with a pluralityof stages of amplification in accordance with the requirements to bemet at a particular substation. One of the intermediate stages of theincoming channel amplifier I I which is coupled to a control circuit has been shown ashaving an input transformer l6, a-vacuum tube II,.and an output transformer I 8. The control circuit is coupled to the intermediate stage ofamplification of the incoming channel amplifier by'the capacitor I0 whichis connected to the make contacts 103' of a relay I00 which is provided with two opposing coils, IOI and I02. The relay 'I00 is furthermore provided with two additional make contacts I04 and I05, thefor'mer contacts being arranged 'to provide a shortcircuit across 'the microphone 'I I, thereby to represent one form of circuit arrangement 'for disabling the outgoing channel amplifier I3.".The-

substation is provided withladial calling device 2| having ofi-normal contacts 22 which will .be closed upon the initial actuation of the'dial device ZI. The off-normal contacts -22 .are connected in parallel with make contacts I05 I of relay I00 and in series with a sourceof voltage 23' .and onewinding I02 of the relay I00. The contacts I03 of the relay I00 are arranged to connect the high-pass filter 23 betweenlgroun'd andthe anode circuit of the vacuum tube'f I. high-pass'filter is so constructedandarranged as to 'transmit-.only the upper range :df voice .ire

.ini'tial actuation of the dial.

quencies so that the control circuit will be responsive to the answer of a called substation. The high-pass filter 23 is coupled by a transformer 24 and a voltage divider to the control electrode of a Vacuum tube 26; The vacuum tube 26 has an anode connected through the winding I| of the relay I00 which may be suitably by-passed by a capacitor 21. The other terminal of the relay winding |0| is connected to the positive terminal of a suitable source of potential for the vacuum tube 26. The capacitor 27 which is connected across the winding |0| of the relay I00 is provided to lower the plate load impedance of the vacuum tube 26 sufiiciently in order to obtain the maximum possible change in anode current with a given applied voice voltage to the control electrode.

When a substation subscriber desires to communicate with another substation, the channel amplifiers I3 and I4 are conditioned for operation by a suitable switch, not shown, which connects a source of 'power to the required power supply. Thereupon the subscriber initiates the outgoing call by actuating the dial calling device 2|, thus closing the ofi-normal contacts 22 upon The contacts 22 being closed complete a circuit from the battery 23' through the winding I02 of the relay I00,

which thereupon closes all of the make contacts I03, I04 and I05. The latter make contact I05 is in parallel with the off-normal contacts 22 of the dial-2| so that the relay is locked into operation. The relay, having operated, has disabled at least one of the channel amplifiers, as by closing of the contacts I04 the input to the channel amplifier I3 has been short-circuited. The control circuit is now also connected to the intermediate stage in the incoming channel amplifier I4 by the closing of the contacts I03 which connect a capacitor I9 to the high-pass filter 23. Since at least one of the channel amplifiers of the associated circuits has been disabled, there is no possibility of interaction between the microphone and the loudspeaker which might bring about a condition of howling during the calling operation.-

The vacuum tube 26 is preferably arranged so that it is biased at or near the plate current cutoff and, hence, the anode current will increase with an increase in applied voice current voltage. When the called substantion responds a portion of the incoming voice frequency energy appearing in the incoming channel amplifier I4 will be transmitted through the high-pass filter 23 to the input of the vacuum tube 26, thus causing increased anode current to flow. The relay winding |0| is arranged to oppose the relay winding I02 and the resulting increase in anode current of the vacuum tube 26 is suificient to release the relay, thereby to restore the disabled channel amplifier to norma1 talking condition and also to disconnect the control circuit from the substation circuit. The high-pass filter 23 is provided so that the control circuit will not be responsive to ring-back tone but will respond only to an answer from the called substation. At the end of the communication period the circuit just described will again be subject to the condition of howl since no means are provided for further disabling any of the channel amplifiers or preventing the interaction therebetween,but this howl now may have a useful purpose since it will remind the substation operator that the equipmentshould be restored to its initial condition.

there is shown a telephone line 30 being connected by a hybrid network or other balancing network 3| with incoming and outgoing channel amplifiers 32 and 33 connected respectively to a loudspeaker 34 and a microphone 35. Each of the channel amplifiers is provided with a plurality of stages of amplification, and of these only an intermediate stage has been shown for the incoming channel amplifier 32. This stage may include an inputtransformer 36, a vacuum tube 31, and an output transformer 38.

The control circuit is arranged to prevent interaction between the incoming and outgoing channel amplifiers during the calling period, and this control circuit is coupled to the incoming channel amplifier by a suitable capacitor 39 which is-connected through the. break contact I22 of a relay I20 to the common conductor between a low-pass filter 4| and a high-pass filter 42, which in turn are each grounded. The low-pass filter 4| is designed to transmit only the lower voice frequencies, in which range are found the dial tone and the ring-back tone. The low-pass filter 4| is connected to a suitable coupling circuit including a transformer 43 and a voltage divider 44 to the input or control electrode of a Vacuum tube 45. The anode of the vacuum tube 45 is connected through a current limiting resistor 46 and one winding I I I of a relay I I0 and the break contacts I 23 of relay I 20 to the positive termina1 of a suitable source of anode voltage. The resistor 46 and the relay winding III are suitably by-passed by a capacitor 48 which lowers the impedance of the plate circuit sufiiciently that the vacuum tube 45 may Reference may now be had to Fig. 2' wherein N be properly responsive to the voice frequency currents which will be impressed upon the control electrode. The vacuum tube 45 is preferably biased at or near the plate current cutoir so that its plate current increases in response to applied low frequency alternating current voltage, thereby to operate the-relay I I 0. The relay I I 0 is provided with make contacts I I3 arranged to disable the outgoing channel, by placing a short-circuit across the microphone 35 or the input tube of the outgoing channel amplifier 33. The relay IIO, therefore, is arranged to become operative in response to the dial tone as soon as the incoming and outgoing channel amplifiers 32 and 33 are conditioned for operation, preparatory to the initiation of a telephone call. The substation is pros vided with a dial calling device 49 which is provided with a pair of off-normal contacts 5| connected in series with a source of potential 52, one winding II2 of relay H0, and a pair of resistors 53 and 54. The common juncture of the resistors 53 and 54 is connected to a grounded capacitor 55. The capacitor 55 is provided so that the circuit including the relay winding II2 and theresistors 53 and 54 will have a time constant sufiicient to hold over operation of the relay I I0 into the period when ring-back tone is impressed through the low-pass filter 4| upon, the control tube 45, whereupon the other winding III of the relay IIO will again become effective to hold the relay I I0 in operated position.

The high-pass filter 42 is connected through a coupling circuit including a transformer 56 and a voltage divider 51 to the control electrode of a vacuum tube58 which has an anode connected tothe winding of the relay I20 which, in turn, is connected to a suitable source of anode voltage. The winding of the relay I20 is by-passed by a capacitor 59 which lowers the impedance ofthe plate circuit sufiiciently so that the maximum possible change in anode current will occur with a g'iven applied voice volta ge. "Therlay I 20 v is provided with make-contact I-2I =and 'break 'con tacts' I22:.an'd I 23. The; make contact I2 I is con-- nected between the ano'cle of the vacuumltube 58 anda grounded-resistor fl. Th break contacts i22sare connected between the coupling'capacitor '39: and the'filters 4 I and Misc 'that-whenlzthe' relay I20 is'energized the control'circuitis disconnected from the communication circuit; "The break contact I23 15 connected between .the source :of anode voltage and the winding) I 'ofirelay :I I sowas'itodeenergize the relay- I I 0 upon the-receipt of "answer from 1a.:called station, thereby to remove the disabling action not this relay from the communication "circuit. v

=Whenever-it is desiredtoinitiate a call asuitable, switch, not shovzngis actuated .to'supplyrthe necessary energy to condition the incoming and outgoing channel amplifiers for operation. JAs soon :as the channel amplifiers :are "conditioned for operation the dial .tone. appearingfacross "the line 30 will be impressed through the rcoup-lingicapacitor39 tothe circuit .betweenlthe low-pass and high-passfilters AI and :42. .The high-pass filter 42 cannot-pass either-the dial itoneor the ring-back tone signals, but thesecurrents are transmitted by th zlow-pass filter-4|. "Therefore, upon .initial :energization of the channel :amplifirsthelow pass filterAIwill transmit dial tone to theinput orthe vacuum'tubetfi which, in'turn, 'respondsbyan increase in anode currentso: as :to I

bringaboutza-ctuation .of therelay H0. The relay I In... having been actuated, disables at least oneof the channelamplifiers bycthe closing of the short 'circuitrcontacts II 3 .connected :across the actuation of the dial Mlhas interrupted the dial tone. The circuitthroughthe relay winding II2v ofthezrlay IIOhas a timeconstant provided by the capacitor .55 and the resistors 53' andt54 such thatthe .relay I I fliremains energized through the dialing operation and; into the periodin'which ring-back tone will be transmittedoverthe line 3'il..to3the incoming channel 'amplifier32. "The ring-back tone isthen :transmitted through-the coupling capacitor and the low-pass filter-'4! to the vacuumitube .45 so that increased anode 'current'again'fiows; thereby to energize the relay winding III of the relay II 0. Thus during the initial-callingoperation at" least one'of the channel amplifiers 'or the'circuits'associated therewith is disabled so :as to-prevent interaction between the incoming and outgoing communication channels.

When the called 'substationsubscriber answers the upper range 'of'voiceiirequencies will be transmitted by thehigh-pass filter -42 lto'the control grid of the vacuumtube 58,? thereby to cause increased anode .current'flow to actuate the relay I20. The relay I opensthenontacts I23, therebyideenergizing the relayI I0:so asto restore this relay to normal condition; thereby removing the disablingtaction brought by the energization of this relay. The entire control circuit isdisconnected from the'communication channels by the opening of the'brea-k contacts I22 ofithe relay .the,winding1of the relay 1 I 20 ,':the.1closeid contacts I2'I and {the current l limiting resistor} 6 I AI'lre control 'circuit therefore is 7 locked rout: of "circuit from cthe' incoming and outgoing communication channels duringthe-ensuing telephone conversa-' tion. At the termination of'thetelephone conversation the. incoming and outgoing communication channels iwill :be subj act 4 to interaction and .howling s0 that the, substation operator will throw 1 the switch to deenergize the power: supply whichvsupplies rthe energy -.to zthe incoming and outgoing channel amplifiers. .eThus the ;.power is'remove'd from the. relay lzllwvhich thereupon is released, thus :restoring the wcontrol .circuit :to its-:originaltcondition.

.LSince telephone subscribers are accustomedrto communicate: over .a system which; is .:simultaneously two-way, thus permitting him toxansvcer promptly zorceven .interruptpa subscriber )Will'i'GX- pect rto estart tconversion'iimmediately tupon hearingtthe:calleclpartyisracknowledgmentof'his call. Thisrequiresrrapidzand positive operation ofrthe relay I"). In .orderathat,this operationzbe both arapidiand positive there is shown a :circuit arrangement intFig. LS-Whichin other :respects iszsimilarto the disclosure of Fig. 2 and,v hence, identical :parts have been given corresponding reference ;.characters. The relay I I0 .inrFigci; in

addition-to the makecontact l'l'3,=is1'provided.with

make .contact I114. :The relay I2l:is provided with make contact -I2I, .break contact I22 and breakemakecontact I 24. The low-pass .filter 4| which energizes-the vacuum tube- 45 supplies energy thereto so: that anode current flows through the :circuit includin na resistor- 46 and one winding .III of the .relay :IIzIl .which is connected throughthe break contacts I24 of-the relay=t2fl to the positive s-ide .ofthe anode potential for :the vacuum tube :45. When the relayz'l Ill-is operated the .disabling contacts I I 3-are closedand,v in addition, contacts I I4 are :closed SO VaS to connect a capacitor 265 throughv a current limiting resistor fiIi-toground. Thecapacitorfiii: is therefore connected in a circuitwhich'receives energy from the fsourceaof anode .potentialfor the vacuum tube 4-5,;therebyto'charge this capacitor to a predetermined potential. During the initial :calling periodtherelay I I0 is initiallyenergized intresponseto the dial tone, land-subsequentlyenergized by the ofienormal contacts ..5 I of the dial device -49, .-and after-the dialing operation again bythe ring-.back-tone through the windin fll II. The capacitor therefore is being charged. during the initial calling period. a 2

When the answer is received fromthecalled substation the'upper. range of voice frequencies is transmitted through the high-pass filterifl to the control tube 5B to bring about actuation ,of

theirelay I20 which locks itself into, .position;-Iby the ,closingof 1 the ,contacts L2 I.

Thetrelay J20 disconnects the control circuit from the communication. channels Icy-opening .the contacts 122. At-the same time the break-make contacts 12-4 are actuatedso that the capacitor 65 is removed fromiacharging circuit and'connected directly across the winding I I I'Of therelay I I0 which just previously was energized by anode current1.from the vacuum tube 45. The capacitorfiS, however, has :been charged to an :oppositepotential-J50 that now current rapidly passes through the winding -I II'ain-the opposite :direction, thereby causing rapid -deenergization and: demagnetization-:of-the relay ;I II}, aso .thatthis relay-rapidly releases-thusuopening its contacts ;I I3. Thereby theudisablingsaction iof ithe relay 5 I I 0 :is removed ina-aztim'ez interval sufficientlyishort: so a that: C0111:

munication' from ':the calling substation may be started immediately in response to an answer received from the called'substation' The remaining operation of the control circuit being identical to the circuit described in connection with Fig. 2, will require no further explanation.

While numerous embodiments of the invention have'been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention.

What is claimed is:

, 1. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line by a balancing network, the combination comprising means including a relay for disabling at least one of said amplifiers during a calling interval, means including a high-pass filter and a vacuum tube arranged to be responsive to voice currents received from the called substation, and circuit means interconnecting said tube and said disabling relay for releasing said relay thereby'to restore said amplifiers to'normal condition.

2. In an automatic telephone substation having incoming. and outgoing channel amplifiers interconnected with a telephone line, the combination comprising a control circuit coupled to an intermediate stage of the incoming channel amplifier, means associated with said control circuit for disabling at least one of said amplifiers during a, callin interval, and means actuated by said control circuit for disconnecting said control' circuit 'in response to an answer from a called substation for the duration of communication between two substations.

3. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line, the combination comprising a control circuit coupled to an intermediatev stage of the incoming channel amplifier, said control circuit including a relay actuated at the beginning of a calling interval for disabling at least one of said amplifiers during the calling interval, a high-pass filter and a vacuum tube arranged to be responsive to voice currents received from the called substation, a circuit means interconnecting said vacuum tube and said disabling relay for releasing said relay in response to an answer from a called substation thereby to restore said amplifier to normal and for disconnecting said control circuit for the duration of communication between two substations.

4. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line, the combination of a dial calling device, means controlled by said device upon actuation of said dial for disabling at least one of said amplifiers, and means responsive to the voice currents received from a called substation for terminatingthe disabling action of said first means. 3 5. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line, the combination of a dial calling device having off-normal contacts, means controlled by said off-normal contacts of said device upon actuation of said dial for disabling at least one of said amplifiers, and means operated responsive to voice frequen cies received from a called substation subscriber for terminating the disabling action of said first means.

6. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line by a balancing network, the combination of a dial calling device having auxiliary off-normal contacts, means including a relay controlled by said ofinormal contacts of said calling device upon actuation thereof for disabling at least one of said amplifiers, and means operated responsive to voice frequencies received from a called substation for terminating the disabling action of said first means by releasing said relay.

7. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line, the combination comprising means responsive to the dial tone for disabling at least one of said amplifiers, means for holding said amplifier disabled during the'calling interval, and means responsive to the upper range of voice frequencies received from the called substation for rendering inefiectiv'e said disabling means.

8. In an automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line, the combination comprising means responsive to the dial tone for disabling at least one of said amplifiers, means including off-normal dial contacts for subsequently holding said amplifier in disabled condition, means responsive 'to ring-back tone for continuing to hold said amplifier in disabled condition, and means responsive to the upper range of voice frequencies received from the called substation 'for rendering inefiective said disabling means.

9. Inan automatic telephone substation having incoming and outgoing channel amplifiers interconnected with a telephone line by a balancing network, the combination comprising means responsive to the dial tone for disabling at least one of said amplifiers, means including ofi-normal dial contacts'for subsequently holding said amplifierin disabled'condition, means responplifier, said control system including means responsive to the dial tone for disabling at least one of said amplifiers, means including oil normal dial contacts for subsequently holding said amplifier disabled, means responsive to ring-back tone for continuing to hold said am plifier in disabled condition, a high-pass filter and a vacuum tube arranged to be responsive to the upper range of voice currents received from the called substation, and means controlled by 'said high-pass filter and vacuum tube for rendering ineffective said disabling means and vfor disconnecting said control circuit forthe duration of communication between two substations 11. In an automatic telephone substation having incoming and' outgoing channel amplifiers interconnected with a telephone line, the combination comprising a control circuit coupled to said incoming amplifier to be responsive to signal currents passing therethrough including lowpass filter means arranged to disable at least oneof said amplifiers in response to a dial tone,

12. In anautomatic telephonesubstation': havingj incoming; and; outgoing channel amplifiers interconnected with. a, telephone line,- the com. bination; comprising a controlcircuit coupled to said. incomnig-lamplifiento :be. responsive to signal currents.- passing therethrough,- said control cir.- cuit. including. arelay: arranged; to disable: at least oneof=said amplifiers, .lowv-pass-filter means arranged.- to be responsivetogdial.toneto: enere gizesaid relay, means for maintaining said: relay energized: during. a. calling interval; and. high- 4 pass. filter means arranged. in responsetothe upper. range-.- of. voice frequencies: received from a called substation-.todisconnect said. control 1 circuit. .zfrorn'v said amplifierthereby. to. restore said amplifiersv to normal. condition;

13. Inan automatic telephone substation-having incoming and. outgoing channel amplifiers interconnected: with' a. telephone line, 1 the come binatic-n comprising a control circuit coupledt'o said incoming amplifier. to be responsive-to signal currents passingtherethrough 1 including a. relay arranged to disable at least one of said ampli-r fiers, low-pas filter means-arrangedto be responsivei to a dial tone to: energize. said. relay; a dial calling. device provided with. off-normal contacts, meansinterconnecting said relay. and

. said; contacts for subsequently holdingsaid. ame

plifier. disabled. during the, dialingv period,. said meansassociatedwith .saidlowr-pass filter means operating. to hold. said. amplifier. disabled; in. re.- sponse to ring-back. tone,. and. highepass. filter means arranged imrespjonse tothe upper. range ofv voice. frequencies received. from a. called: sub;- station. todisconnect. said. control. circuit from said amplifier thereby to restore said' amplifiers to normal condition.

14. .lnlan automatic. telephone substation having. incoming and outgoing channel amplifiers interconnected with. a, telephone line, tliecomlbination. comprising. a. control circuit. coupled. to said incoming amplifier. to.be. responsive to. sig: nal currents passing. therethrough, including. a

plural winding .relay arrangedi to.disable. atileast one of; said amplifiers, low-pass filter, means arrangi -d to. be. responsive. to.v dial. tone to energize one. winding. of, saidv relay, a dial. callingv device provided with. ofinormal. contacts, a second .WindingofL said. relaybeing controlled? by. saidcontacts. .for. subsequently. holding .said' amplifier disabled. during .the dialing. period,.. said. first .re.-

lay I winding associated with. said low-pass. filter meansheing, operative ,to..hold .said relay in energizedpositionin responsetoring-back.tone, and

highepass filter. m,eans.,arranged in. response to the; upper. range of. voice. frequencies received from .a.called substation to .deenergize said relayto. disconnect said .control circuit from said amplifier. thereby. to. restore, said'. amplifiers. to normal; condition.

agsnegma 15:. In: anpautomaticzt'eiephone substaticmhavs in'gincoming and. outgoing channel; amplifiers interconnected. with; a. telephone: line; thee. com. bination comprising a control.circuitcoupledxto said incoming: amplifier: to: be responsiveto" sigie nal currents. passing. therethrough; including a relay arranged .to disabl'epat. least one: ofasaid amplifiers; low-pass: filter' means: arranged: to be responsive. to dial tone; to. energize said; relay; a dial :calling :device provided: with off "-normal cone tacts, means controlled by said contacts fOlfSLlbe sequently holding said amplifier" disabled: during the dialing period; said -.low passr filter-J means being operative v to hold said relayinc energized position in. response to: ring-back tone,- a'. second relay'connected to be: energized? from night-pass filter means inzresponseto? the" upper: ranges of voice frequencies? received from: as called: substattion; means oontrolled-bysaidilatter-relayzdor. connecting said: control. circuit" from". said am:- plifien and meansicontrolledsbyisaiddatterrelay forsrapidly. releasing :said firstrre-laqv.

16. an automatic telephonesubstation have ing incoming: and". outgoing: channel. amplifiers interconnected; with. a telephone line, the combination; comprising a control circuit coupled to saidfincoming amplifier to be responsivesto-signal currents passing therethrough including a re? lay: arranged to-disable at leastoneof said .am* plifiers,.low-pass filter meansarranged'to be*responsive' to dial tone to energize said relay, means for holding said; relay in. energized: condition throughout. a. calling; intervah. a second relay connected; to I be energized from: highepassi: filter means": in: response to: the: upper range; ofi voice frequencies received: from. a called substation, means controlled by. saidlatterr-relay for discon necting; said; control. circuit from. said, amplifier, a capacitor arranged to." bercharged .by. operation of saidfirst. relay, andpmeans controlledv by said second; relay." for: discharging said; charged capacitonthro'ugh saidrfirst relay tov insure rapid release thereof.

1'7 In. an automatic. telephone substation .hav ing incoming: and outgoing. channeli amplifiers interc0nnected:.with a telephone line, the combination comprising a'control circuitcoupled to said incoming amplifier-to. be-responsiveto sig: nal currents passing therethrough, including a relay arranged todisableat least one 05- said amplifiers, low-pass filter means arrangedto be responsive: to dial tone to energize said relay,-.a dial. calling device provided withofi-norma-l contacts, means controlled by said contacts forvsubsequentlly. holding said. amplifier disabled during the" dialing period, said low-pass filter. means being operativetohcldsaidgrelay energized position ini.response:-touring-back .tone; a second relay: connected to be: energized fromahigh-pass filter. means in response: to the. upper rangeof voice frequencies received from an called substation, means controlled by'said latten'relay for disconnecting saidcontrohcircuit. from said am.- plifier; a capacitor. arranged- 1301 138 chargedl by operation ofssaid' first. relay," and :meansccontrolled by said .latter relay for discharging: saidfcharged capacitor through said i first relay to insure rapid release thereof."

RQ'SWEEIL Hi HERRICKL' 

